Electrical heating elements of refractory material are known for use in very high temperature furnaces. One known type is shown in U.S. Pat. No. 3,859,501 and comprises a helical silicon carbide element having a small gap between the turns of the helix. A voltage differential exists across the gap and can be of sufficient magnitude to cause a voltage discharge especially in the presence of contaminants which condense or otherwise become disposed in the gap. The helical heater construction is, in addition, structurally weak.
Another conventional refractory heater, known as a Norton DU heater, employs two parallel rods of silicon carbide each having a high resistance portion and a low resistance portion. The high resistance portions of the parallel rods are, in operation, disposed within a furnace chamber and are connected at their ends by a connecting block of silicon carbide. The connecting block is of a size and configuration to require a relatively large opening in the furnace wall or roof for insertion of the heater into the furnace chamber. Moreover, an insulative two-hole plug must be precisely mated to the heater to retain the parallel rods within the mounting opening in the furnace. Such parallel rod construction is also subject to the deleterious effects of unequal bending stresses during furnace operation. Furthermore, the connecting block is of substantial mass, such that if the heater is suspended from the roof of a furnace chamber, the heater can be subject to pendulous movement which can cause bending stresses and cracking of the heater rods.
A coaxially constructed heater is shown in U.S. Pat. No. 3,764,718 of specific design for use in a vacuum furnace and includes a tubular resistor element and a coaxially disposed inner resistor element connected at one end to the surrounding tube. The inner and outer elements are in primary embodiment of the same resistive material which are stated to be carbon, silicon carbide, metal or metal alloy, and both inner and outer elements serve as heaters, the inner element radiating heat through the outer element which also radiates heat from its surface. In another version of this heater, the outer element is operated primarily as a conductor and as a radiant element for heat generated by one or more inner resistor elements. The heater is moveably mounted above a vacuum chamber and the outer element is adapted to be disposed within the chamber for radiation from its entire surface.